Elastic polypropylene was first isolated by solvent extraction of polypropylene obtained by Ziegler-Natta polymerization of propylene (inter alia, U.S. Pat. No. 3,175,999). A process for preparing primarily isotactic, fractionable, elastomeric polyolefins has been described (U.S. Pat. No. 4,335,225), using a catalyst prepared by reacting a solid support such as alumina with, e.g., a tetraalkylzirconium compound. U.S. Pat. No. 5,594,080 discloses preparation of a stereoblock elastomeric polypropylene using a metallocene catalyst containing two unbridged phenylindeniyl liganids connected to zirconium , i.e. (phenylindenyl).sub.2 ZrCl.sub.2. Chien et al. (J. Am. Chem. Soc., 112, 2030 (1 990); macromolecules, 25, 1242 (1992); macromolecules, 24, 850 (1991)) prepared a thermoplastic, elastomeric polypropylene containing about 30% crystalline isotactic polypropylene, using a metallocene catalyst.
Collins et al. (Macromolecules, 28, 3771 (1995); Macromol. Symp., 98, 223 (1995)) describe the use of zirconium and hafnium metallocene catalysts to prepare elastic polypropylene. These polymers had 29-54% mmmm pentads, i.e. they were predominantly isotactic. Significantly, elastic polypropylene could only be obtained when the molecular weight was greater than 50,000 and when the mmmm content of the polymer exceeded 38%, using exclusively hafnium complexes.
European Patent Application No. 666,267 describes the use of a metallocene catalyst of the type {fluorenyl-C.sub.2 H.sub.4 -indenyl}ZrCl.sub.2 to prepare polypropylene in the presence of hydrogen. Polymer properties are not described. The same catalyst was used (Organometallics, 13, 647 (1 994)) to polymerize propylene in toluene in the absence of hydrogen; highly isotactic polymers having mmmm contents ranging from 38 to 64% and melting points of 104-110.degree. C. were obtained. When the C.sub.2 H.sub.4 bridging group was replaced by --SiMe.sub.2 -- (U.S. Pat. No. 5,391,789), only a viscous, tacky oil was obtained.
European Patent Application No. EP 707,016 describes a catalyst system comprising {2-Me-4-naphthyl-indenyl-SiMe.sub.2 -fluorenyl}ZrCl.sub.2, a Lewis acid compound and an organoaluminum compound, to prepare elastomeric polypropylene. U.S. Pat. No. 5,516,848 describes a process for making elastomeric polypropylenes employing a mixture of two different metallocene catalysts, one of which was a monocyclopentadienyl transition metal compound.
Metallocene catalysts generally can be defined as catalysts comprising one or more cyclopentadienyl groups, including substituted cyclopentadienyl groups, in combination with a transition metal, typically a metal of Group IVB of the Periodic Chart (i.e., titanium, zirconium, and hafnium). Useful metallocenes have been described comprising two cyclopentadienyl-type rings, in which both rings are identical (symmetrical metallocenes) and in which the two rings are different, due either to differing substitution patterns on identical ring systems or to the presence of two different ring systems (unsymmetrical metallocenes). The first reports of the use of cyclopentadienyl metallocenes to catalyze the polymerization of I-olefins appeared in 1957. British Patent Application GB 934,281 first described metallocene catalysts of the type (R-indenyl).sub.2 MX.sub.2 and (R-fluorenyl).sub.2 MX.sub.2, where R represents an alkyl or aryl substitLutent on the aroniatic ring, M represents titaniiumi zirconium or hafnium, and X represents a halide or an alkyl or alkoxy group, or the like, having from 1 to 12 carbon atoms. Since then, numerous patents and other publications have provided ample evidence of the commercial interest in these materials.
Four limiting cases illustrate the possibilities for stereoregularity in polypropylene. The isotactic structure is typically described as having the methyl side groups oriented so that they are all on the same side of the plane containing the polymer backbone. Another way of describing the isotactic structure employs Bovey's NMR (nuclear magnetic resonance) nomenclature: An isotactic diad, represented by m (for meso), is a pair of propylene units placed in the polymer chain so that both methyl side groups lie on the same side of the plane containing the polymer backbone. Similarly, a mmrnrn pentad represents five monomel units with their methyl groups all oriented the same way. .sup.13 C NMR can be used to determine the relative amounts of the various pentads present in a propylene polymer.
In contrast, in the syndiotactic structure, the methyl groups are oriented alternately above and below the plane containing the polymer backbone. A pair of propylene units with this up-down arrangement comprises an r (for racemic) diad, and rrrr would correspond to five similarly arranged monomer units.
Hemi-isofactic polypropylene is similar to the isotactic kind except that every other methyl side group has a random orientation.
Finally, in the atactic polymer, the orientation of the methyl groups with respect to the plane containing the polymer backbone is random and the number of r and m diads is the same.
The physical properties of polypropylene depend greatly on polymer stereoregularity. For example, isotactic and syndiotactic polypropylene are crystalline polymers that are insoluble in hydrocarbon solvents such as cold xylene. Because of the presence of crystallites that scatter light, they are often opaque or translucent. Additionally, these polymers are stiff, inelastic, and high-melting, their melting points being 171 and 138.degree. C., respectively.
In contrast, hemi-isotactic and atactic polymers are non-crystalline and soluble in cold xylene. Atactic polypropylene is transparent, flexible and elastomeric.
Because isotactic and syndiotactic polypropylenes are such useful materials, their preparation has been the object of extensive research. For example, catalysts that produce isotactic polypropylene are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403; catalysts that lead to syndiotactic polyolefins are described in U.S. Pat. Nos. 3,258,455, 3,305,538, 3,364,190, 4,892,851, 5,155,080 and 5,225,500.
The precise effect of changes in catalyst structure on catalyst activity and polymer stereoregularity is difficult to predict. For example, U.S. Pat. No. 5,459,218 describes catalysts of the type {flu-bridge-Cp}ZrCl.sub.2, wherein flu=fluorenyl, bridge=Me.sub.2 Si or Ph.sub.2 Si, and wherein Me=methyl, Ph=phenyl, and Cp=cyclopentadienyl. It is broadly stated and claimed that the propylene polymers contain at least 50% rr content, whereas the syndiotactic content actually achieved ranged from 74 to 82%. The highest molecular weight given for the propylene polymers is 66,000. No unusual properties of these polymers were disclosed nor were any clues provided as to how the catalysts could be modified to produce polymers having a lesser or greater syndiotactic content. By comparison, U.S. Pat. No. 4,892,851 describes a catalyst of a very similar type, namely {flu--CMe.sub.2 --Cp}ZrCl.sub.2, which is reported to produce even more highly syndiotactic (92% rr) polypropylene.
A relationship between catalyst symmetry and polypropylene stereostructure has been proposed (Trends in Polymer Science, 2, 158 (1994)). Symmetry elements, typically described in terms of "point groups" of a bis(fluorenyl)MX.sub.2 -type catalyst are shown in FIGS. 1 and 2 (comparative). FIG. 1 illustrates the different symmetry elements that can occur in metallocene catalysts and FIG. 2 provides illustrative examples. A catalyst (FIG. 1 and, e.g., FIG. 2 entry 20) having point group C.sub.2v, has three symmetry elements: the structure is unchanged after a 180 degree rotation about the bisector of the MX.sub.2 plane and there are two mirror planes of symmetry, reflections in which leave the structure unchanged. One contains the MX.sub.2 plane .sigma..sub.h and the other is orthogonal to and bisects the MX.sub.2 plane .sigma..sub.v. Looking straight on at the MX.sub.2 plane, one sees that the top and bottom as well as the left and right hand side of the catalyst molecule are the same. A catalyst 21 having point group C.sub.2 (entry 21 in FIG. 2) contains only a C.sub.2 axis: the top and bottom of the molecule are the same, so that 180 degree rotation about the bisector of the MX.sub.2 plane leaves the structure unchanged. Catalyst 22 having point group C.sub.s contains only a mirror plane of symmetry orthogonal to and bisecting the MX.sub.2 plane. Viewed as described above, the left- and right hand sides of the catalyst are the same but the top and bottom are different. Catalyst 25 of C.sub.1 point group symmetry contain none of these three symmetry elements.
According to FIG. 2, catalysts of point group C.sub.2v are said to produce atactic polypropylene, except that Cp.sub.2 TiPh.sub.2 (24) atypically produces a stereoblock isotactic polymer (perhaps by a different polymerization mechanism) at low temperatures. Metallocenes having C.sub.2 and C.sub.s point group symmetry are said to produce isotactic and syndiotactic polymers, respectively. However, U.S. Pat. No. 4,769,510 describes catalysts of the type {ind-bridge-ind}MX.sub.2, which exists in two isomeric forms. One form is chiral andic consists of a d,l pair of enantiomers; it produces isotactic polypropylene. The second (meso) form is achiral and produces atactic polypropylene (Organometallics, 14,1256(1995)). Catalysts with C.sub.1 symmetry are said to produce either hemi-isotactic 23 (in FIG. 2) or stereoblock isotactic-atactic 25 (in FIG. 2) polypropylene.
European Patent Application No. 537,130 describes the effect of the size of an organic group attached to a cyclopentadietyl ring in metallocenes of tlhe type {Cp-CMe.sub.2 -flu}ZrCl.sub.2 (C.sub.s point group symmetry), which was shown to produce syndiotactic polypropylene. Introduction of a methyl group into the 3-position of the cyclopentadienyl ring led to {3-MeCp-CMe.sub.2 -flu}ZrCl.sub.2 (C.sub.1 point group symmetry), which produced a hemi-isotactic polymer. When the 3-CH.sub.3 group was replaced by a t-butyl group to form {3-t-BuCp-CMe.sub.2 -flu}ZrCl.sub.2 (also C.sub.1 point group), isotactic polypropylene was obtained instead.
U.S. Pat. No. 5,459,218 relates to syndiotactic polypropylene prepared using silyl bridged metallocenes and having molecular weights up to 66,000. U.S. Pat. No. 5,668,230 discloses certain specific ethylene bridged fluorenyl-containing metallocenes to catalyze olefins polymerization.
One skilled in the art can only conclude that there are no reliable, general correlations between the structure of a metallocene catalyst and the stereospecificity or the stereoregularity of polymers produced thereby.